1. Field of the Invention
The present invention relates a biomedical member made of sintered ceramics, a method for producing the same and an artificial joint.
2. Description of the Related Art
Alumina-based ceramics and zirconia-based ceramics are bio-inactive materials and have excellent properties such as high mechanical strength and high wear resistance, and therefore have been used in medical applications such as artificial joint and artificial dental implant. In order to make an artificial hip joint, for example, a ceramic is used to form the head and an ultra-high molecular polyethylene is used to form the acetabulum socket, because the combination of alumina-based or zirconia-based ceramics and ultra-high molecular polyethylene is less susceptible to wearing and damage than in the case of metals (for example, refer to Japanese Patent Examined Publication No. 6-22572).
An artificial hip joint having sliding members both formed from alumina-based ceramics has also be developed (for example, refer to Japanese Unexamined Patent Publication (Kokai) No. 2000-16836).
Much attention has also been paid to a composite ceramic formed from alumina and zirconia mixed in predetermined amounts, since such a material achieves higher strength than those of individual ceramics, due to the effect of forming fine crystal grains (for example, refer to Ryoichi SHIKATA et al. “Mechanical Properties and Characterization of ZrO2—Al2O3 Composites with High Fracture Strength”, Japan Society of Powder and Powder Metallurgy; Apr. 10, 1991; Vol. 38, No. 3, pp. 57-61).
For the purpose of reducing the production cost or improving the property of the composite material described above, researches have been conducted on adding small amounts of additional components. For example, it has been disclosed to make it possible to sinter dense material at a lower temperature by adding SiO2, MgO and CaO to a composition containing 70% by weight or more alumina, thereby to make a material having excellent wear resistance at a low production cost (for example, refer to Japanese Unexamined Patent Publication (Kokai) No. 5-206514 and Japanese Unexamined Patent Publication (Kokai) No. 9-221354).
It has also been disclosed to produce alumina-zirconia composite material having high toughness, by adding an oxides of the group 5A metal of the Periodic Table and SiO2 together, thereby making use of the effect of accelerating the anisotropic growth (for example, refer to Japanese Unexamined Patent Publication (Kokai) No. 2000-159568).
Alumina-based ceramics is an excellent biomedical member, but is far inferior to zirconia-based ceramics with regards to strength and toughness. In the case of an artificial hip joint that has a sliding interface between alumina-based ceramics members, for example, cases of patients whose artificial hip joint were broken due to insufficient strength and toughness of alumina-based ceramics have been reported.
Zirconia-based ceramics has higher strength and higher toughness than alumina-based ceramics, but is susceptible to phase transition in an in vivo environment containing abundant water, thus resulting in poor surface roughness. Poor surface roughness leads to wear in the sliding surfaces thereby generating particle of the worn ceramic. When the particle of the worn ceramic is accumulated in the body tissues of a patient around the artificial hip joint, bone resorption takes place. The bone resorption causes loosening between the artificial hip joint and the bone. Generation of the wear particle occurs particularly conspicuously in the sliding interface between zirconia-based ceramics members.
With regards to the composite material described above, it is known that fracture toughness is improved but strength and hardness are caused to decrease by the generation of grains having shape anisotropy. While improving the fracture toughness requires it to grow the grains having shape anisotropy in elongated shapes, strength and hardness decrease as the grains grow larger. According to Japanese Unexamined Patent Publication (Kokai) No. 2000-159568, while anisotropic growth of alumina grains showed the effect of improving the toughness, bending strength decreased to 1050 MPa or lower, indicating that the growth of grains having shape anisotropy resulted in a decrease in strength. Therefore, it is necessary to study a method for improving toughness while suppressing the crystal grains from growing, in order to obtain a material having high strength and high toughness.